Process of enameling



Patented Nov. 16, 1937 umransrares- A N OFFICE a No Drawing. ApplicationJune 28, 1934,

Serial No. 732,895

2, cum in m -way This invention relates to the m 'ofeiiameli'ng ferrousmetal, and is particularly concerned with an enameling iron and a newand improved' method of-enameling such iron which prevents the formationof bubbles and similar imperfections in enamel coatings on ferrous metalarticles.

The usual practice in coating ferrous sheet metal with enamel is asfollows: A ferrous sheet metal article to be enameled is covered withenameling material either as a liquid or in a finely divided form, andthe article is then placed in a suitable furnace where it is heated atabout 1600 F. for a. short period of time, for example from about threeto five minutes, and until the enameling material has fused to form alayer, covering the surface of the metal. The article is then removedfrom the furnace and cooled to room temperature and a second layer ofenameling material is placed on the article and it is then reheated toabout 1500 F. The article is then allowed to, cool, which step completesthe enameling operation.

During the first heating, small bubbles form within or beneath themolten enamel layer and often rise thru it and break, at the surface.This action is commonly referred to as the primary 1 boiling. Before theheating is completed these bubbles have ceased to form and hence theenamel 3 layer is substantially smooth and quiet when the article isremoved from the furnace. During the first to 45 seconds of the secondheating bubbleslagain form and often break thru the ename 35. "reboilingor "secondary boiling".

Many eflorts have been made to prevent the primary and secondaryboiling, but, so far as I am aware, little success has been attained byanyone prior to my invention. It is desirable 40 that these boilings orthe formation of bubbles should be avoided, since the bubbles cause anon-uniformity in the thickness of enamel and often cause thin spots inthe enamel coating. Where a"white finish coating of enamel is providedon the metal the bubbles resulting from boiling often cause black specksor discoloration of the finished enameled surface.

I have discovered that the boiling and reboiling action can be largely,if not entirely, overcome and have devised a method by which this resultmay be attained.

The metal ordinarily enameled is ferrous sheet metal containing carbonor carbides, or both. I have found that when such metal is so treatedthat deoxidation of the iron by carbon or car- This action is commonlyreferred to as j bides'is prevented during the enameling operation anduntil the enameling material has formed a protective layer over themetal, surfaces the amount of boiling andreboiling can be greatlyreduced, if not actually eliminated entirely. Ac- 5 cordingly, myinvention is based on this discovery and may be practiced in any one ofseveral manners, including the protection of the metal by (a) providingthe metal with a surface layer of substantially pure iron, (b) coatingthe metal 10 with an air-excluding soluble silicate, such as sodiumsilicate, or (c) deeply pickling the metal. After the metal has beentreated to protect it as aforesaid, it is covered with enamelingmaterial, heated for a suitable time at about 1600 R, cooled l to roomtemperature, recoated with enameling material, and then reheated toabout 1500 F., and finally cooled.

Generally speaking, the heating and cooling operations are similar tothose heretofore used but, 20 according to the present invention, arecarried out on metal which has been protected as aforesaid.

, In practicing my invention by the step of providing the article with asurface layerof sub- '25 stantiallypure iron, I may proceed along eitherof two courses: The ferrous metal sheet or article is .freed from greaseand other impurities in the ordinary well known manner, and is then madethe cathode in an electroplating bath, the 30 electrolyte of whichpreferably consists of a normal solution of neutral ferrous sulphate.The

anode may be of any suitable ferrous material.

'A, current is used of a strength which will give "a bright, firm,closely adherent, smooth plate of substantially pure iron on thearticle. The thus coated ferrous metal article, when enameled accordingto the ordinary practice, was found to exhibit substantially no tendencyto boiling or rebelling during either of the two 'heatings, and 40 theenameled layer was found to be of substantially uniform thickness and tobe free from all occluded bubble spaces. Moreover, the enamel coatingwas tightly adherent and showed no tendency to flake or chip off.

Alternatively, the ferrous metal sheet or article may be heated in anoxidizing furnace between about 1000 F. and about 1200 F. until an oxidecoating has been formed thereon; or, the article may be subjected to theaction of steam at a temperature between about 1100 F. and about 1600F.; or, if desired, oxidized sheets discharged from a normalizingfurnace may be employed- These oxidized sheets or articles arethenheated in a reducing furnace to reduce the oxide surface layer tosubstantially carbon free iron. The sheets or articles to be enameledand having such a layer of carbon free iron on their surfaces are thensubjected to the ordinary enameling operation described hereinabove.

It is also-within the purview-of my invention T '7 e I the time thearticle is placedin the first heating to provide an enameling sheetwhich has been rolled from an ingot in such a manner as to retain on thesurface thereof the layer of substan-' tially pure iron which is formedabout the ingot during the initial steps in the cooling of 'the ingotafter the same has been poured.

In practicing my method byusing a soluble silicate, as above mentioned,the ferrous sheet metal article-to be enameled isfirst cleaned in theusual manner, for example, by subjecting it to an alkali cleaning bath,rinsing it, acid pickling it, rinsing it again and dipping it into aneutralizing bath, after which it is immersed in a boiling 6% solutionof'a soluble silicate, for example, sodium silicate. After beingsubjected for about two minutes in this solution, it is then dried atabout 250 F. Then the surfaces are covered with the enameling materialand the article subjected to the usual enameling operation abovementioned. Substantially no boiling action was observed during theheatings and the enamel coating which resulted was of substantiallyuniform thickness and substantially free from discoloration of theenameled surface.

In practicing my invention by the method including the pickling stepabove mentioned, the ferrous metal article to be enameled is pickledpreferably in concentrated hydro+chloric acid at a temperature of about100 F. for about one and one-half hours. remaining thereon isneutralized in the ordinary manner. The article is then enameled in theordinary manner, the amount of boiling being greatly reduced, althoughnot to the extent attained by the other methods herein disclosed. Ibelieve that pickling in concentrated hydrochloric acid removes asubstantial amount of the surface carbides and to that extent decreasesboiling but since the pickling does not effect the car- Then it isrinsed and the acid bon, the boiling traceable to the carbon is notavoided.

I believe that the primary cause of boiling and reboiling is due to theoxidation of the iron in the surface portions of the ferrous articlebeing enameled, ,which, oxidation. takes place between furnace and thetime the enameling material -'melt's. I believe that such iron oxidereacts with the iron carbides and carbon in the metal to produce 'a gas;probably carbon monoxide or carbon dioxide, whichbeing liberated beneaththe liquid enamel coating forms bubbles, some of which rise to thesurface and break and others of which are entrapped in the enamel. Atany rate, by protecting the metal from deoxidation or by the deoxidizinginfluences of carbides or carbon until the enameling material has meltedand formed a protective layer thereon, the production of bubbles can belargely, if not entirely, avoided. I

I have set forth several specific illustrations of my invention, so thatthose skilled in the art may be able to practice the same. The scope ofmy invention is defined in what is claimed.

What I claim is:"

1. In the process of enameling ferrous sheet metal those stepscomprising heating the metal in an oxidizing atmosphere to form asmooth, firm oxide scale thereon, heating the oxidized metal in areducing atmosphere to remove the oxygen and provide a layer of carbonfree iron on the base metal, thereafter working the metal to the desiredtexture and shape, and enameling directly to the prepared surface of thebase metal whereby an improved bond results with an absence ofreboiling.

2. The process of enameling which includes'the steps of removing carbonand carbides from the surface film of ferrous sheet metal containingcarbon and carbides by subjecting a surface of said metal to oxidizingand then reducing conditions, applying enameling material to said film,and heating said metal and enameling material to enameling temperatures.

' KARL KAUTZ.

